feat(frontend): add former name display, referral discount type, and refunds to bookings

Update frontend types, stores, and components to support name history display and new API fields.

- Add previousFirstName/previousLastName to User type and BookingUser type
- Add referral discount_source type and refunds array to Booking type
- Create nameDisplay.ts utility for rendering '(formerly ...)' labels
- Create booking.ts utility for booking-related helpers
- Update 25+ admin, payments, today, and account components to display
  former names on booking cards, modals, appointment views, and user lists

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

frontend/src/routes/admin/notifications/+page.svelte

@@ -331,7 +331,7 @@
 	function getNotificationSubtitle(n: Notification): string {
 		const parts = [formatRelative(n.created_at)];
 		if (n.user_name) {
-			parts.push(n.user_name);
+			parts.push(n.user_name); {/* TODO: add formerly name when previous name data is available */}
 		}
 		return parts.join(' — ');
 	}
@@ -427,7 +427,7 @@
 				</p>
 			</div>
 		{:else}
-			<div class="space-y-3">
+			<div class="space-y-3 {loading ? 'opacity-60' : ''}">
 				{#each notifications as n (n.id)}
 					{@const actionable =
 						!n.acknowledged_at &&